Light-emitting elements (organic EL elements) including organic compounds and utilizing electroluminescence (EL) have been put to more practical use. In the basic structure of such a light-emitting element, an organic compound layer containing a light-emitting material (an EL layer) is provided between a pair of electrodes. Carriers are injected by application of voltage to the element, and light emission can be obtained from the light-emitting material by using the recombination energy of the carriers.
The light-emitting elements are self-luminous elements and thus have advantages over liquid crystal displays, such as high visibility and no need for backlight when used as pixels of a display, and are suitable as flat panel display elements. In addition, it is also a great advantage that a display including such light-emitting elements can be manufactured as a thin and lightweight display. Furthermore, an extremely high response speed is also a feature thereof.
In such light-emitting elements, light-emitting layers can be successively formed two-dimensionally, so that planar light emission can be obtained. This feature is difficult to realize with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources applied to lighting devices and the like.
Displays or lighting devices including light-emitting elements can be suitably used for a variety of electronic devices as described above, and research and development of light-emitting elements have progressed for higher efficiency or longer lifetime.
An organic compound having an acceptor property is a material for a hole-injection layer that is used to facilitate the injection of carriers, particularly holes, into an EL layer. The organic compound having an acceptor property can be easily deposited by evaporation and thus is suitable for mass production and has become widely used. However, the injection of holes into an EL layer is difficult when the LUMO level of the organic compound having an acceptor property is distanced from the HOMO level of an organic compound included in a hole-transport layer. In contrast, when the HOMO level of the organic compound included in the hole-transport layer is raised so as to be closer to the LUMO level of the organic compound having an acceptor property, the difference between the HOMO level of the light-emitting layer and the HOMO level of the organic compound included in the hole-transport layer is large, causing difficulty in hole injection from the hole-transport layer into a host material in the light-emitting layer even when holes can be injected into the EL layer.
In a structure disclosed in Patent Document 1, a hole-transport material whose HOMO level is between the HOMO level of a first hole-injection layer and the HOMO level of a host material is provided between a light-emitting layer and a first hole-transport layer in contact with the hole-injection layer.
Although the characteristics of light-emitting elements have been improved remarkably, advanced requirements for various characteristics including efficiency and durability are not yet satisfied.